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Repairing a degraded hardware RAID 5
 
A RAID 5 becomes degraded when one disk fails. While it continues to operate normally except for somewhat slower performance, a degraded RAID 5 is not protected from a second hard drive failure. Actions should be immediately taken to rebuild it. However, if a mistake is made or an error occurs during rebuild, data may be permanently lost. A safe strategy is to make copies of the most critical files before rebuilding.

A RAID can be configured with a "hot spare" in which case the RAID controller will start the rebuild automatically. No human intervention is required other than replacing the failing drive.

The rebuild process varies slightly among RAID controllers. Generally, it consist of the following steps:
  • Identifying the failing drive. Usually a status light (amber or red) will help to locate it. Be aware that disconnecting the wrong drive will break the RAID.
  • Replacing the failing drive with a new drive.
  • Use a vendor-supplied utility or invoke the RAID BIOS to rebuild the RAID. In the latter case, the computer will need to be shutdown.
When the rebuild is complete, the RAID status is back to normal.

As simple as it seems, complication from attempts to rebuild a RAID 5 is by far the most common reason for a RAID 5 to become broken.